The presence of sodium, sulfur, and other corrossive contaminants in fuels burned in turbine, as well as reciprocating engines, causes expensive maintenance problems and shortens engine life. Turbine engine fuel contamination problems are found in land and marine equipment, especially in middle east installations. Significant problems from contaminated fuel are also seen with turbine engines used for propulsion of ships. Fuel contamination results in corrosion of turbine engine parts, especially at high operating temperatures. High temperature corrosion results from the presence of vanadium, sodium and potassium. These elements form compounds during engine operation which form deposits on components, dissolve coatings, and leave parts open to sulfidation attack.
Generally, refiners of hydrocarbon fuels employ processes to remove sulfur and metal ion contamination from middle distillate hydrocarbons which are essentially procedures where the hydrocarbon is first treated with water (steam). Water is employed to extract water soluable contaminants from the hydrocarbons. Water, along with contaminants, is then removed by using centrifugal methods, water coalescing techniques, passive gravity (settling) methods, and filtration. There are inherent inefficiencies in these multiple steps and reduction of water, sulfur and metallic contamination to levels that meet standards and specifications of fuel users is often difficult and expensive.
Once purified at the refinery, middle distillate hydrocarbons often become recontaminated enroute to the point of use. Salty or contaminated water is often co-mingled with middle distillate fuels during transportation and storage. Users of middle distillate fuels, such as those operating turbine engines, must maintain special fuel treatment and purification equipment to protect their engines from damage. Most turbine engine manufacturers specify liquid fuel purity requirements. Turbine engine fuels such as liquid petroleum gas, light virgin naptha, heavy virgin naptha, kerosenes, diesel fuels and gas oils are required to contain less than 1% by weight free water and less than 0.1 parts/million of vanadium, sodium, potassium, calcium and lead. Copper must be below 0.02 parts/million. Consequently, these engine fuels must undergo purification processes that assure users that damage to internal critical engine parts is minimized.
Prior art decontamination techniques often include multiple steps for the purification of middle distillate fuels both at the refining stage and at point of use. These prior art techniques such as centrifugal methods, coalescing processes, passive gravity settling and filtration do not always efficiently separate water and contamination from the hydrocarbon. A particularly difficult separation problem results from the oil/water interface between the water hydrocarbon phases. Centrifugal and passive gravity settling techniques cannot efficiently deal with this interface and some cross-contamination frequently occurs between the water and oil phases. Coalescers can handle only certain maximum water volumes in petroleum hydrocarbons before they become overpowered by entrained water and fail. Conventional filtration, of course, is not capable of separation of water from hydrocarbons at all.
The present invention provides a simple, efficient technique wherein water from any source is co-mingled with hydrocarbons, either intentionally or inadvertently and can be removed from the hydrocarbons with a hollow fiber cross flow membrane system.
The former cross flow systems using hollow fiber membranes required a pump which acted against fluid resistances generated by the piping between the pump outlet and the entry to the separation module including the hollow fiber membrane. Loss of fluid from inside to outside of the hollow fibers, such as by permeation, causes a reduction in volume and resistance that is counter balanced by the resistance subsequently generated by the hollow fibers themselves and by the piping during the return of the retentate fluid back to the reservoir of the system.
Contaminated fluid would be fed by a pump to a membrane cross separation model. Permeate would be conducted to an engine or a reservoir and retentate would be recycled back to a open reservoir. The reservoir would be drained of settled contaminants. The reservoir would feed into the contaminated fluid line being pumped directly to the membrane cross separation module for recycling and further decontamination.
A further disadvantage of this system is the use of the reservoir. A detection system must be employed with a reservoir in a cross flow system, such as float valves, for monitoring high and low fluid levels. This is required so that small changes in flow either to the system or out of the system do not cause the reservoir to either over flow or cause the pump to run dry. This necessary feature adds to the cost and reduces the reliability of the overall system. Further, as water builds in the reservoir, a proportionate reduction in permeate flow rate occurs unless this water is removed from the system. In many cases, the volume of water is not large enough per unit volume of fluid being decontaminated, but there are occasions when the large water volume can enter the system and cause a significant permeate flow rate reduction. This represents a problem if the system is hooked directly to a jet engine fuel supply.
Another disadvantage of the use of reservoir in cross flow separation systems is the inconvenience of removal of water and particulate contamination in a continuous manner as it builds up in the retentate fluid during operation.
The present invention provides a decontamination system which further eliminates the reservoir entirely and incorporates significant improvements in system efficiency.
Inherent in the reservoir type systems is an inefficient use of the entire length of the hollow fiber membranes. Applicant has found that the portion of the hollow fibers proximate to the separation module inlet of the system meet a higher pressure head then those portions at the distall end of the housing fibers proximate to the separation module output. The present invention further provides means for increasing the efficiency of the entire length of the hollow fiber membrane thereby increasing the permeate output of a separation module unit.